Automatic gain control system



Sept. 16, 1952 s. F. SPENCER AUTOMATIC GAIN CONTROL SYSTEM 3 Sheets-Sheet 1 Filed NOV. 22, 1948 /f LE np 0 R .c f mnl* cn 0, 7./ wu am L ,Aw n... M n n pa u s 6/ m www. 2n mw a M4. m .vlulL i a. L w Ew. 1 5 fm l WJ 4h C 7 F F. 1 5 .M an .c .m H ,l R m L 1 2f M L+ N u B we 0 c a. a) mm zw fm ma 0 46C VL TGE R TN. n ,Mm MFR/,wm M6. #m J @M Sept. 16, 1952 B. F. SPENCER 2,611,081;

' AUTOMATIC GAIN CONTROL SYSTEM l Filed Nov. 22, 1948 3 Sheets-Sheet 2 if L? F.' AMPLI/:VERS

Sept. 16, 1952 B. F. SPENCER AUTOMATIC GAIN CONTROL SYSTEM s sheetlsneet s Filed Nov. 22,- 1948 hihihi HUD/0 MPL /F/ER E AMPLI/VERS INVENTOR 55m/,0MM SPENCER BY #wir ATTORNEY patented Sept. 16, 1952 UNITED stares Parent ori-Tice;

Benjamin F;

Spencer, Garden City, N. Y., assignor Delaware:

application .November 22, 1948", serial No. 61,361.5

' This invention' relates tdautomatifcfgain controlsystems'and in particular to amplied'jauto# maticgain controlsystems. Amplied. automatic .gain control. (AGCY sysL-' tems or automatic volume control (AVCYJsystems' are well known'in. the art. 'IngeneraL' the ainpliication o ftlfiaAGfC voltage is obtained" di.` rectl-y by a. direct-coupled ampli'ercoupled sov as to amplify the 'y unidirectional output voltage1 from the AGC- rectiiying. device'or the i' amplil cation is :obtained indirectly` byproviding radio# frequency oraudio-frequency'amplification precedingthe AGC rectifyingY device.

While these prior systems have been .satisfactory in many. specific applicationajth'ey possess certain limitationsaanddisadvantages. For ex;-v

ample; a. direct-coupled amplifier coupledto the output circuit of tl1e.AGCrectiiying4 device requiresa stable-sourceof negativesupply voltage to set'the potentialsof the. direct-coupled ampli.- er-in order to provide a controlv-oltage thatis negative with respect to ground.- Sources ofA- negative supply voltageare oftenexpensive .to provide land'fliflicult to justify especiallywhen, as is usually theA case, the directfcoupled amplifier is. the onlystage requiring. a,Y source'of l negativevoltage. Furthermore, it is .dii'cultto design -a direct-coupled amplifier thatA isY un.

affected Yby power supply. voltagevariations. andi tube changes.

YAmplified AGr'C"s'ysten-1s,4 employed 1 inv radio' receiving systems, in which the amplification isf obtainedindirectly byv an audio-freiiuency or vsig#- nal intelligence amplierf followingj the second.

detector but. preceding..` the AGCl `rec'ztifyin'g"de; vice are adversely. aiete'd by "variations of" the' percentage of'modulation. Wave.` Variations. ofthe percentage' of.. moduff` lation result in corresponding.variations oftheVA AGC'vltage. This type of'AGCfsystemjisthus practically limited to radio systems'employing-'aj' constant percentage `of1`modulation, andy isv not" widely used. l Y A One of thefmost'satisfactory amplifiedAQC systemsof the prior" art isithe typeemployin'g an additionallsta'ge 'of Aradio-frequency" 'or interi mediate-frequency ampli'cation" preceding the` AGC rectifying device in" Which' theamplicati'om inthe additional stage' is obtained` at thewsame" frequency" as the amplifiers that 4are controlled.

This type of ampliedAGCsystem has beengen'- erally applied" to' andinavigation type'receiving eauiprrlents;sA I-I o iv'v ever', in certainultra-liighffrequencyv (UHF'` of tnereCeiV'ed 'carrier high quality communication f @claims (c1. 25a-zo) centrating: and

communication" and navigation types, thisampliied AGC system has not been all together satisfactory. Forinstancain UHF navigational receivers of the instrument landing type" in which no suitable methodis Aknown for v'obtainingamat'the' frequency of 'the UHF' signaliwave tolb'e `received; the lampliiication or"v thesignal Wave is by necessity'.Qbtained, after;

plilc'ation directly heterodyning in1 a`converter'stage, byja cascade seriesof;intermediatefrequency amplifier stages. In j such ai receiver Jthere'exists an' upper limit toitlic totaljam'ount of stable amplicationob tainablein the I. F.' amplifier system. In practice in s'uch'a receivergth'is upper limit is junavoidably atel Withinput signal voltages 'as small as. al' l fraction of.. a. microvoltv and must'produce an` output signal voltage of several volts' toY the. seconfdfdetec'tcil4 An additional'stag'e oiamplica.- tion preceding. atfthe same frequency. as that'of `^the-II F. am

pli-rsystem providesa total ovrall ampli-A cation greatly in excess of this upper'limit andthere results a. condition of highly undesirable f' lregenerative Afeedback producing intolerable infstabilityin therover-all I. F. amplication system.l 1

Tourther complicate the gainy control problem, a-receiver of the instrument landingv type must;

operate overa. range ofv radiated signal intensity.

varying vfrom a.- ievv` micromicrowatts. atv vgreat distances from; the instrument landing transgreat radiated v.signal mitting, antenna to a very intens'A y attthe landingpoint; produced by a highl gain electromagnetic parabolic horn .con-

focusingenergy directly in this landingurcgona Such. a receiver to .be at .all usable must possess a smoothly acting, high-gain amplified AGQ system to prevent over-'loading of-.fthe I..F.amplier stages. and in addition toiication from-:becoming overloaded. n

An importantyobf'ect of the invention isvto improveftlfle;StabilityV of amplifiers in Which'amplifiiedtautomati'c gaincontrol isvrequirecl,v

Anotherffobjectiis to provide an amplied .auto-- maticfgai'n control system'whichiis not susceptiblel tcioverloading...

additional object is 'to' provide ianY amplifiedy automatic gainycontrolz system. with increased?v amplificati'onloi the AGC voltage: l f A ivfurther object -is to provide .ani amplified.. automatic `vgain control system .capable 'ofi' highlyy stable performance over a very wide-rangeoii the AGC rectifier, and operating iprei/.@4111the amplifier providing theAGQ ampli introduced an improved amplified automatic gain control system in which the amplification of the AGC Voltage is effected through the use of an amplifier preceding the AGC rectifying device, this amplifier operating at a multiple of the frequency of the amplifier stages to be controlled.

The above objects and general description of the present invention will be more fully understood and further objects and advantages will become apparent from a study of the following detailed description, in connection with the drawings, wherein:

Figure 1 is a block diagram of a radio receiver employing the invention.

Figure 2 is a schematic diagram illustrating one embodiment of the invention.

Figure 3 is a schematic diagram of a second embodiment of the invention employing a pushpush frequency doubler amplifier.

Figure 4 is a schematic diagram illustrating an alternative arrangement of the circuit of Figure 3.

Figure 5 is a schematic diagram illustrating another embodiment of the invention.

The receiver block diagram of Figure l, which may be for example the .block diagram of' a navigational receiving system, shows an antenna I coupled to a single-pole double-throw switch 2. One terminal of switch 2 is coupled to mixer 3 and the other terminal is coupled to the first I. F. amplifier 5. Local oscillator 4 is coupled to mixer 3. Three stages of identical I. F. amplifiers 5 are coupled in cascade forming block I I, the last I. F. amplifier stage being coupled to second detector 6. Audio amplifier 1 is coupled to second detector 6 and utilization device- 8 is coupled to audio amplifier 1. This utilization device may be, for example, a crosspointer meter as employed in instrument landing receivers or a position plotting mechanism as employed in range and azimuth radio navigation systems. Harmonic amplifier 9 couples the output of the last I. F. amplifier stage of block II to AGC rectifier I0.

The amplified AGC voltage from rectifier I0 is applied to each I. F. amplifier stage of block II and in addition to audio amplifier 1. The band-pass harmonic amplifier 9 provides amplification of the output voltage from the I. F. amplifiers II, this amplification being obtained at a selected harmonic frequency of the I. F. amplifiers II. Withl switch 2 'positioned so that antenna I couples to mixer 3, the system of Fig. 1 operates as a superheterodyne receiver. If the switch is thrown to transfer the antenna directly to the input of the I. F. amplifiers II, the system then operates as a fixed tuned radio-frequency receiver.

The schematic diagram of Fig.v2 shows a parallel resonant circuit comprising condenserA I2 and primary inductor I3 coupled to the last I. F. am-V. plifier of block II. The lower terminal junction of this resonant circuit is returned to the B+ terminal through resistor I4 and is bypassed to ground by condenser I5. A conventional second detector circuit comprising a diode detector tube 20 associated with a'parallel resonant circuit of secondary inductor I 6 and condenser I1 is transformer coupled to primary inductor I3. The signal intelligence voltage produced across diode load resistor I8 in parallel with filter condenser I9 is coupled through a second filter consisting of resistor 2| and condenser 22 to the audio amplifier 1.

The upper terminal junction of Ythe parallel resonant circuit of condenser I2 and primary inductor I3 is capacity coupled to the control-elctrode of a band-pass harmonic amplifier tube 25 through condenser 23. Resistor 24 returns the control-electrode to a common grounding terminal. Bias for the harmonic amplifier tube 25 is provided by cathode resistor 26 in parallel with bypass condenser 21. The screen-grid of tube 25 is bypassed to ground by condenser 28 and is coupled to the B+ terminal through resistor 29. The plate or anode of tube 25 is coupled to the B+ terminal through load resistor 30. Condenser 3I bypasses the B+ terminal to ground. The plate of tube 25 is capacity coupled to the plate of the AGC rectifier tube 31 by condenser 32. A parallel resonant circuit consisting of inductor 33 and condenser 34 is coupled from the plate of rectier tube 31 to one terminal of rectifier load resistor 35. The other terminal of load resistor 35 is coupled to the cathode of tube 31 which is returned to ground.

A unidirectional AGC voltage produced across load resistor'35 is filtered by condenser 36 and is coupled through resistor 38 to a Voltage divider comprising resistors 40 and 4I. kAn additional filter condenser 39 is coupled in parallel with the voltage divider; The unidirectional'AGC' voltage across the divider is coupled over conductor lead 42 to the gain controlling electrodes of the I. F. amplifiers of block II. A second AGC unidirectional voltage reduced n magnitude by the v'oltage divider is coupled from the junction of resistors 40'and 4I over conductor lead 43 to the gain controlling electrode of audio amplifier 1.

The conventional second detector associated with diode tube 20 operates in the usual manner. The transformer coupled resonantcircuits consisting of condenser I2 and primary inductor I3 and secondary inductor I6 and condenser I1 are tuned to the center frequency of the pass bandvof the I. F. amplifiers II. The band-pass harmonic amplifier associated with tube 25 is biased almost to cut-off by cathode resistor 26. Voltages of the pass-band frequency of the I. F. amplifiers coupled through condenser 23 to the control-electrode of tube 25 cause amplifier pulses of plate current to flowv through resistor 30. These pulses of plate current couple through condenser 32 to excite the resonant circuit consisting of inductor 33 and condenser 34. This parallel resonant circuit is tuned to a selected harmonic frequency of the center pass band frequency of the I. F. amplifiers II. The harmonic amplifier produces in its output circuit an amplie ed voltage at a frequency nfl with respect to the I. F. amplifier outputv voltage at frequency f1, where n'is an integer greater than unity. This amplified output voltage at the frequency nfl is rectified by rectifier 31 and produces a unidirectional AGC voltage across load resistor 35. This AGC voltageis further filtered and applied over conductoriead 42 to` the gain controlling electrodes of the I. F. amplifiers II and is also applied through a voltage divider to the gain controlling electrode of audio amplifier 1.

One important characteristic of this amplified AGC systemis that the total amount of stable amplification at a single frequency fr may be obtained in the I. F. amplifiers II while the amplification of the AGC voltage isV obtained by the harmonic amplier 9 operating at a harmonic frequency nfr of the I. F. amplifiers. With this arrangement there is no regenerative feedback fromtheoutput of the harmonic amplier to the input yof theI. F. amplifiers 'to cause undesirable instability. vThe full limit of the total amount of 5;. stableVV amplification at ai; ,Single freclllilily1-Y 1"' which is approximately.,1.000.000; canfthuS-,be provided in the I. F. amplifier system..` Anygdesired'almount of amplication'of rthieiAGC -ylae maynow be provided by.thefharmonic.ampliiie1 9 :at `a1frequencyfnfr Without impairing.: thefslacf.- bility Aof the I.1-F. amplifier systerncll. A suitablej harmonic 1 amplier operating.; as 1 a frequency s doubler can -readily; provide au Voltage 1 ganxfas: hghaslO. l0

Another important charactersticfof thisffamf; plied AGC system, is that for s trcng-z1.,` Ffoutnut; voltagesapplied to thecontrokelectrnde;of :therharmonic amplifier tube. 2 5f, an.; additnnalr-.ba'sz-z voltageis developed acrossgresistorldue to cene 1 trol-electrode current flow.: and Athe f: harmQncq amplifier operatesias a vclassgCstage,.;v vit1 1 .asrerf sultineincrease ineiaencygdue ftcfaymoreoptif mumane1epf'p1ate .currentlow- The..distortion o thexvoltaec `Wave atxfthe entrblrlelectrode of: tuba-2 5. .due to controlrelectrede:current flow for.. Strong-voltage v,vi/aves does-@not .cause they varrlpliicationof `tube .25;inzthef2usual sense; as-the., tube is Operatingas 'aV-frequency multiplier While @conventional ampliergtubebeccmes sa Y urated due-to control-electrodecurrentnorma for example-v alimiter tubeginfa;frequencymoulu lation receiver, ,andi its output.;evoltagezremains Substantially constant '-.upon futher increase the amplitude of the input; voltage, thezereuuenjcy3o.if multiplier stage doesznot beeomeisaturatedrduerg, to a distorted control-electrode voltagea- Af.dis. 1 tcrtedmcontrolfelectrode..voltage is, rich in fhar-:f: monicifrequencies one. zof; which; is .selected fby the itunedroutput circuit of thefrequency multie.l plier stage. As a matter ofifact;Lthisgdistortion': together With the correspondingimprovement in the, angle of. platey V'current now; producesan; in creased amplication inrtube :25 -for strongjyolt-e,l age ,-.WaV eS This; is `aiyery ;useful;;result.;. for -it tends. to produce; a 'greater AG C;..1voltage just; 'f when a greater AGC voltage yis. required ;as con-: trasted to certain prior art systems in which the AGC system becomesoverloadedat this Vlevelandx. the AGC system loses; itsiacontrol iover ',thergainzfq ofthe ampliiersr- Y An additional characteristic. I of` rthisyamplied 1 AGC system is thatfa high gain'rnayfbeobtained.. inthe harmonic amplier 9 ifdesiredrnlnaconf. ventional R.v F. or I. F.1i..amplifier ;stage,=.-fthe..v.alf;z50 lowable] amount of stable..axnpliiication.lobtaineif`A able fin; axsingle .amplifier tube l iszlimited :by the e' amount: of undesired feedback through fthe: platen.l tn control-electrode capacity of the tube. To .ob'i'v l tain high gain.; conventional' R. Fi and amai plifier-Q stagesemploypentode...tubes,V with* very small; capacity between: plate sand. control-'ele'ctrede.;- ,Howeven lfor a harmonic *amplifier stage in which the output circuitzzof the=iampliflerfisf tuned to a harmonicfzthenput frequencythe 60' alleWable y amount; of stable.V amplification for; w a .singleamplier tube isV much higher, than:A fori f a Acom'eiitional RrF. or I.:F.-ampliiier tube-as no undesirable feedback ,exists .L from vplate tofcon- 1 trolf'electrode. Thispllallowable?, gain :isf/very high l for the-harmonic amplier and asl a result.' l triode Iype :tubes: .withi a v.very much` larger lplateffI to controlelectrode .capacity :than pentodescan I1 be.readily,employed.:` n A; f

An illustrationof;.za;triode `tube embodiment l is shown; in the schematic diagramofdlig; 3; in* which a parallel; resonant, circuit comprising asecondary inductor-f IIlLIv andi a. condenser -I 02 is' transformer 1 coupled .c to; primary ff-in'cluctorlfik I 3f.- Resister I-Il3 1coupleszthe center tappi-secondary@ .z is-coupled topone 'control=electrode and 'theflowe refmencion-:10i toftherotor roi tbe-fspliti-stater-co' denser I-o2.;- Thev rotor f ofithis. c ondenserri i coupledo 12a ,commOIl ,.groundinggterminah )Tn uppenterminaliiunctioniof :the-@resonantcircrn terminal Ej unction isA coupled-atai-second contro electrodeiofsv a--dual .triode tube "HM-The cathod ofitube lll4fislcoupled througlr a-.para-llell com bination,v ,ofliresistor I 05 andncondenser.- iffIIIBfrtQ? ground.: Thertwoiplatesiof-itube'f I 04 areecoupledr together. :and to the upper terminal `junction a paralleliresonant circuit vof lcondenser: I 0];zr`e e sistor IllA and 'jprimaryrinductor-.Ilz VThe lowent: terminal .-junction Lof this resonantcircuit ls coupled to. vthe.-l3".i-. terminal fand. isralsoilbypassedto ."ground. by. the condenser` 3 I. Another Aparall resonant circuit.consistingfof-v 'secondaryindctorlf- I Ill andy condenser III is? transformeracoupledito primary inductor rI 09.M The uppertermina'h jun'caff:

tion orw this resonant circuit is coupled 'tofone tera'.

minal of AGC rectifier I I2.'- the-otherrterrninalofl thejrectie'r being coupled to ground.l The lo'weniv terminaljunction of this` resonant circuit-,rigen coupled fto load 'resistor 35 inV 'parallel'with"cone--l 1 electrodes of the I. Framplie'rs jI I and their duced AGC voltage at the junction ofthe dividere' resistors-III) and 4I, isA coupled over `lead^43-to the audioampiinerm' TheV circuit arrangement :of "triodef tubell'f" serves a-dual purpose.-v Itis a-combinationfull-x Wave 'infinite impedance detector 'andlpush'pushife frequencydoubler ampliiien a] combination well suited for "-thisapplicationr* The control-ele trodes of tube IIM are vexcitedfronfr the cent'e tapped resonant` circuitlgofl secondaryfI induct'or II'II- and' condenser'IZ Vtunedto resonance Nfetff the intermediate frequency. Cathode tresistov I05-i`s l of'vsuch fval'ueas tobias the;;control' electrodes -falmostfj to .-cutoiiA C'ondenserMI'II5'` L'in g parallel-f with 'resistor `IIll' jbypas'sesfthe vcathode j to ground at the intermediatefrequency." `The detected signalA Aintelligencel y Voltage[developed across cathode resistor" `I 05 is coupled 'fthrongtr);` the filter-of lresistor` lfandfcondenser" 22to" essentially bypassed l'toground atgaudio";'=fre=*^- quencies' duetothegsmallvalue of inductancelr of primary `inductor?ICIQ land the `'large-ivalli of' capacity of condenser 3-I.'V `The circuit-thu functions as a=fullwave1innite impedance-ade tectoit Thecircuitalsofunctions 4-asha pushy push frequency doubleramplier as 'the .re'so nant circuit of 'condenser |01 landfprimaryf-in ductor |09` ist'uned to twice ythe requencyfof'f theV I. F. ampliers.- TWO pulses of vplatefculr rentiiow -throughthis resonantl circuit 'forf eac cycle-of the 1E. outputV voltage. "This y*type of frequency-doubler `ampliiier lis/more 'elci'en than l the previous harmonic! -freduency-mult' plierv `'amplifier land va -Isomewhatl higher fvoltag gain may be obtained; TheVAGC rectierifIIZ-il' coupled Vto this push-push frequency f doubler The circuit arrangementofV VFigi "4 femploys tube-|04 as a combination full-wave* grideleak detector y and push-push frequency .doubler lam'- I pllfieizl The cathode of the tube |,04.is` grounded..

The center tap of'secondary inductor,- l|0|.isre-' turned to `ground through a large grid-'leak re-f sistor. 202. l Condenser 20| bypasses; the.: center t'apof secondary inductor lutto-ground at .thel

intermediate frequency.. Thelplatesfof tube' v|04 are-coupled together and` are returned -through load; resistor 30 to. the. .B+ terminal.: The platesof tube |04 are :coupled also througligconand .condenser l I.

circuit fintheiconventional manner; Inthe full-wave grid-leak detector, rectification; takes placein the gridcircuit .with the grids and cathode functioning as a full-wave' diodeV detector and with; the 'rectified'. voltage being developed. across the grid-leak resistor 202 in parallel withcondenser 20|.v 'Ihe-signalfin-` telligence voltage developed across this grid-leak f resistor is4 not amplified by tube: |04 in the usual sensebut is filtered by resistor 2| and condenser' 22 andcoupl ed to audio amplifier 1. The pushyv pus h f 'requency doubler amplifier functions as previously described providingan amplifierput:

put voltage at a frequency of 211 withrespect to .theI F. amplifier output voltage at` the frequency f1. AGC rectifier' ||2 coupled to the.

output circuit of the frequency doubler amplifier supplies a unidirectional gain control voltage tothe LF. amplifiers and the audio amplifier as previously described. Y

An arrangement in which the amplification' of the AGC voltage is obtained after frequency doubling is illustrated by the schematic diagram in Fig. 5. A duo-diode-triode tube 303 is employed as a combination full-wave diode rectifier ordetector" and grounded-'grid band-pass amplifier. -The two plates of thediode section of the tube are coupled to the resonant circuitof secondary inductor |0| and condenser |02. rotor of the split-stator condenser |02 grounded. The control-electrode of thetube is also coupled to ground. The cathode is returned to ground through resistor304 and is unbypassed. The value of resistor 304 is such as to bias the control-electrode almost toont-off.

Full-wave rectified 4I. F. voltage appears across cathode resistor'304. YVThis rectified voltage is filtered by resistor 2| and condenser 22` to remove Vthe intermediate frequency voltages and thek audio v.signal intelligence voltage is applied to audio amplifier l.; The rectified I. F. voltage across resistor `3||4.drives the cathode of the grounded-grid amplifier at twice the intermediate frequency as two pulses ofV cathode current flow for each cycle of the I. F. output voltage.. The resonant circuit of inductor ||0 and condenser is tunedv to twice the intermediate frequency.A The cathode driven, grounded-grid Aamplifier provides the desired amount of voltage vgain at the frequency 2h. The AGC rectifier ||2 coupled tothe output circuit. of tlieground-grid amplifier supplies a unidirectional voltage to the gain controlling electrodes of the I. F. amplifiers and the audio amplifier 1 as previously described.

While the invention has been described in general terms, it is not specifically limited to the present disclosure. The invention is readily adaptable to other modifications and may be Thel arranged in combination.v with known circuits to form more useful apparatus. Whatis claimed is: Y 1.'The'combination ofa band` pass amplifier Adesigned to provide substantially linear amplificationof an amplitude modulated carriersignal of a selected frequency and to 'attenuate signale'y of' all other frequencies, utilization means .in-

cluding an amplitude modulation detector.- and means for directly applying said amplified signal with its modulation, Without the introduction of amplitude distortion, to said 'amplitude m'odulation detector; means responsive to the outputlof said amplifier to produce and select a'signalpair of electron discharge'tubes each includingat least a cathode, a control grid, and an anode. mean'svconnecting said cathodes to each other and means connecting said anodes together-to place the space discharge paths of said tubes in parallel, means including a .center-tapped resonantcircuit for applying a modulated carrier signal to said control-grids in phase opposition, va load resistor connected between the center tap of said resonant circuit and said cathodes, and modulation Vfrequency utilization means 'connected across said load resistor; a gain control rectifier, and means including afband pass filter, designed to pass a harmonic of said carrier signal and to attenuate all other signals. coupling said anodes to said rectifier.

3`. The combination of a band pass amplifier including a gaincontrol terminal and designed to amplify an amplitude modulated carrier signal and to attenuate signals of vother frequencies, 'a pair vof electron discharge devices having respective' cathode means connected to a common cathode. terminal, respective anode meansA connected to a common anode terminal, and separate signal input electrodes, means including a center-tapped resonant circuit for applying said amplied modulated carrier signal to said signal input electrodes in phase opposition, a load resistor connected between the center tapiof said resonant circuit and lsaid common cathode terminal, 'and modulationifrequency utilization vmeans connected across said load'resistor; a gain'controlv rectifier having rinput. and output terminals, means including a band ypass filter, designed to pass substantially only a:.h'armonic of said-carrier signal, couplingisaid first mentioned common anode terminal tothe input of said rectifier, and means coupling the output of said rectifier to said'ga'in control terminal. f

4. A combined amplitudemodulation detector and gain control circuit, includinga pair of diodes having separate anodes and 'respective cathode; means connected to .a common A'cathode terminal, and an amplifier .tube having a'cathode connected to said common cathode terminal, lsaid amplifier tube having a separate anode and at least one control grid, means including a center#y applying a modulated'l mon cathode terminal, and modulation frequency' utilization means connected across said load resistor;- means connecting said control grid direct? ly-, to said center tap, a control rectifier having 9 input terminals and output terminals, means including a band pass lter, designed to pass substantially only a harmonic of said carrier signal, coupling said amplier tube anode to said rectier input terminals, and gain control signal utilization means connected tothe output terminals of said rectier. j

, BENJAMIN F. SPENCER.

REFERENCES CITED v The following references are of record in the le of this patent:

UNITED STATES PATENTS Name 'v Date Number Y Nichols June 29;-1922- Number Number Name Date MacDonald Feb. 28, 1928 Eaves May 7, 1929 Wheeler Sept. 3, 1935 Schlesinger Jan. 26, 1937 Stone Dec. 12, 1939 Weagant May V27, 1941 Travis Aug. 19, 1941 Bailey Apn 2, 1946 Koch May 6, 1947 Berger Oct. 31, 1950 FOREIGN PATENTS Country Date GreatBritain sept'. 1o, 1931 

